Graham Singleton wrote:
>
> >A dipole for the xponder wastes a theoretical one half of the power
> >radiating above the aircraft, meaning that a 1/4 wave antenna will
> >have more gain in the desired direction.
>
> Fred
> I'm not a radio ham or xpurt but my understanding is that the dipole is
> better than the 1/4 wave because its radiation pattern is more like a
> horizontal doughnut, whereas the 1/4 wave is more or less a hemisphere. The
> 1/2 wave dipole puts more of the energy out horizontally.
> I've used a Bob Archer dipole, neat printed circuit board with the antenna
> etched onto it, and it worked well from inside the fuselage.
> Someone correct me if that is not correct?
> Graham
That's my understanding too, 'cept it's in the hemisphere below the
aircraft where ATC's transponder is. The dipole is better
horizontally, only if the poles are straight, and the theoretical
advantage at an unrealistic 0-degrees is supposed to be .85dB - not a
lot. And often less relevant than other characteristics, with plusses
and minuses to both types.
It's worth noting that no transponder I know of pumps out anything
near the minimum xmit power permitted by the gubment (70W in the low
altitude structure), and they further allow for 50% loss in the coax,
usually requiring malice to achieve. It's logical to assume that
ATC's equipment is designed to tolerate even less signal strength,
meaning there's a lot of "slop" in the system. So the fact that a
given antenna works fine means it's at least radiating a small
fraction of the xponder's 200-250W in the right direction. It may
coincidentally be performing better than another - or not!
Regards,
Fred F.
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